1. Field of the Invention
The present invention relates to a failure diagnosis apparatus for diagnosing the failure of an evaporative fuel processing system which temporarily stores evaporative fuel generated in a fuel tank and supplies the stored evaporative fuel to an internal combustion engine.
2. Description of the Related Art
A failure diagnosis apparatus for an evaporative fuel processing system is shown in Japanese Patent Publication No. 3199057, for example. According to this apparatus, a negative pressure is introduced into the evaporative fuel processing system through a purge control valve from the intake pipe of an internal combustion engine. When the pressure in the evaporative fuel processing system does not reach a predetermined negative pressure within a predetermined time period, the purge control valve is determined to be abnormal.
In the above-described conventional failure diagnosis apparatus, it is necessary to close a valve provided in the air passage which introduces air into the evaporative fuel processing system, in order to negatively pressurize the inside of the evaporative fuel processing system. Accordingly, the failure diagnosis cannot be performed when performing the ordinary evaporative fuel purge from the evaporative fuel processing system to the intake system of the engine. Therefore, if the failure diagnosis is performed at an appropriate frequency, the evaporative fuel stored in the evaporative fuel processing system may not be sufficiently purged. In other words, there is a case where the failure diagnosis cannot be performed at a sufficient frequency, when performing the purge of evaporative fuel at an appropriate frequency.